1. Field of the Invention
Example embodiments of the present invention relate to wireless communications systems. For example, at least some example embodiments of the present invention relate to methods and apparatuses for providing a scalable spectrum, spectrum bandwidth and/or throughput to users in a wireless communication system.
2. Description of the Conventional Art
Code division multiple access (CDMA) techniques (e.g., IS-95, cdma2000, Wideband CDMA (WCDMA), etc.) employ code channels for transmitting information to multiple users simultaneously. Each code channel is distinguished by a unique spreading code (e.g., Gold codes, Walsh codes, OVSF codes, etc.). WCDMA is a 3rd Generation wireless technology that utilizes a higher spectrum bandwidth than, for example, IS-95 CDMA. With the increased spectrum bandwidth, WCDMA provides higher data rates or throughput to wireless users. Universal Mobile Telecommunications Systems (UMTS) may utilize WCDMA as a suitable transport mechanism.
FIG. 1 illustrates a portion of a CDMA wireless system referred to as a CDMA cell site or cell. As shown, the CDMA cell site includes a base transceiver station (BTS) 110 and a radio interface part. The BTS 110 may communicate with a radio network controller (RNC) (not shown) as is well-known in the art. The combination of one or more BTSs and an RNC is referred to as a radio access network (RAN).
The BTS 110 may include multiple radio transceivers for communicating with the RNC and a plurality of users UE1, UE2, UE3, . . . UEn via the radio interface part. As used herein, the term “user,” may be synonymous with mobile station, mobile user, user equipment (UE), subscriber, wireless terminal and/or remote station and may describe a remote user of wireless resources in a wireless communication network. For example, user equipment may be a mobile phone, wireless equipped computer, wireless equipped personal digital assistant (PDA), etc.
Referring still to FIG. 1, BTS 110 and the plurality of users UE1, UE2, UE3, . . . , UEn may communicate in the forward link (e.g., from BTS to user) or the reverse link (e.g., from user to BTS) simultaneously via code channels. The code channels in the forward link, for example, may be differentiated from each other by a unique spreading code (e.g., a Gold code, Walsh code, OVSF code, etc.). A unique spreading code may be assigned to each user by the BTS 110. The code channels in the reverse link may also be differentiated in a similar manner.
Combining data signals with a unique spreading code spreads each individual data signal over a much wider spectrum (e.g., 5 MHz for WCDMA) than the spectrum (e.g., 15 kHz for speech data signal) occupied by the data signals prior to spreading. After spreading, the BTS 110 combines all spread data signals, and transmits the resultant signal to each user served by the BTS 110. Each user is informed of its assigned spreading code via a separate signaling channel as is well-known in the art. Spreading the data signal over a wider spectrum allows for reduced transmission power while still obtaining a suitable data rate and/or throughput.
In one example, upon receiving the resultant signal transmitted from the BTS 110, user UE1 identifies the data signal intended for user UE1 using the same unique spreading code used to spread the data signal at the BTS 110. Other data signals spread using other spreading codes are seen by user UE1 as noise. The length of a spreading code assigned to each user is dependent upon the information data rate assigned to the user and/or the spectrum capability of each user. For example, the wider the spectrum capability of the user, the longer the assigned spreading code will be for the same information data rate. This is a result of the larger spectrum over which the data signal may be spread.
Referring still to the above example, at user UE1 the de-spread data signal is sent to a filter that allows the energy associated with the received data signal to pass, while reducing the interference. The signal-to-noise ratio (SNR) is determined by the ratio of the data signal power to the sum of all of the other signal powers. The SNR is enhanced by the processing gain, that is, the ratio of the spectrum over which the data signal has been spread to the baseband data rate.
To provide higher throughput and/or information data rates required by wireless applications, for example, on the forward link, the BTS 110 and users UE1, UE2, . . . , UEn may operate in a larger spectrum (e.g., 10 MHz or 20 MHz). Conventionally, these higher spectrum bandwidth requirements are satisfied by allowing users to access multiple carriers (e.g., multiple 5 MHz carriers in WCDMA) simultaneously while maintaining each individual carrier structure. This is referred to an Nx system. An Nx system allows backward compatibility with existing systems. However, utilizing an Nx system may increase the cost of radio-frequency (RF) design for users due to added RF path components and/or may introduce cross-carrier interference due to imperfect filter response in single carrier frequency designs.